1. Field of the Invention
This invention relates to apparatus for use in testing printed circuit boards or similar devices and, more particularly, to an improved means of supporting a printed circuit board or similar device in a predetermined location within a fixture utilised to effect electrical connections with the test point zones of the device. The improvement extends both to assuring better alignment between the individual contacts of an array of some carried by the fixture with the test point zones of the device to be tested and to facilitating the introduction of successive devices to be tested into the fixture.
2. Description of the Prior Art
The type of fixtures commonly employed in the testing of printed circuit boards or the like employ a pair of relatively shiftable supports, one typically in the nature of a base and the other in the nature of a head mounted for reciprocation toward and away from the base, with one of such supports carrying a matrix of electrical contacts having electrical leads associated therewith for coupling with external electrical testing equipment, while the other support is adapted to receive a succession of printed circuit boards to be tested in a position such that each of the test point zones of the printed circuit board under test will be aligned with a corresponding one of the contacts carried by the other support, so that each test point zone will be properly engaged by a corresponding contact to effect a temporary electrical connection therebetween when the two supports provided by the fixture are relatively shifted toward each other into an operative relationship thereof.
Since the individual test zones of a printed circuit board are relatively small and frequently relatively closely spaced, and since the contacts are also relatively closely spaced, it has long been a problem to assure sufficiently precise alignment between the respective test point zones of the device under test and the corresponding contacts provided in the fixture for engaging such zones. With older types of printed circuit boards and fixtures for use in effecting electrical connections with the test point zones thereof during testing, it was commonly regarded as sufficient merely to define a predetermined location for the device to be tested upon one of the support portions of the fixture by means of locating pins, stop blocks or the like rigidly secured to that support portion of the fixture, while taking reasonable pains in the construction of the fixture to assure that the path along which the support portions of the fixture would be relatively shifted toward their operative relationship for bringing a contact into engagement with each of the test point zones of the device under test would be relatively free from substantial lateral deviation.
In the testing of printed circuit boards of the complexity and degree of miniaturization that has now become typical, however, it is not unusual to employ matrix arrangements of contacts spaced on 0.05 inch centers, and a lateral deviation from precise alignment between individual contacts and corresponding test point zones of the device under test of as little as one or two thousandths of an inch from perfect alignment between all of the multitudinous test point zones of the device and the intended corresponding contacts can be sufficient to prevent the electrical connections required for satisfactory testing from being accomplished. Although in theory it would be possible to so construct the fixtures that the relative movement between the two support portions thereof would be constrained to linearity within a tighter tolerance than that mentioned, the expense involved in fabricating and maintaining such fixtures creates a geat need for a better solution to the problem. It may also be noted in passing that, when similarly tight tolerances must be held between the printed circuit board under test and locating pins or blocks rigidly secured to one of the support portions of the fixture within the confines of the latter, the emplacement and removal of successive printed circuit boards to be tested is necessarily also rendered more difficult and time consuming.
One of the more recent approaches to coping with the tighter alignment tolerances now required in the testing of printed circuit boards of modern type has involved the provision of elongate alignment pins fixedly mounted on the support portion of the fixture that will hold the printed circuit board being tested, by means of fitting locating holes in the board over such pins, and with a distal portion of such pins being precisely tapered and adapted to be received within corresponding holes in the other support portion of the fixture. Although such last-mentioned technique has been reasonably satisfactory in assuring relatively precise location of the printed circuit board being tested upon one of the support portions of the fixture and also in providing a means independent of the involved primary guide structures for urging the two support portions of the fixture into predetermined alignment as they are relatively shifted into their operative relationship for ffeecting the electrical connections required for testing, such approach to the problem has inherently involved certain practical disadvantages, including the sizes and types of parts requiring precision machining initially and frequent replacement as wear occurs, as well as imposing serious limitations upon the speed and convenience of emplacement and removal of successive printed circuit boards to be tested within a fixture that is so constructed.